Possible Clues:

• Sadly his mad idol is primly fastidious
• His mad idol turns out to be prim and proper
• 23 down perhaps is hard, as it describes one who hasn't met their match
• Card game is going to his head? It explains why one never had any mates!
• Primly fastidious
• Over-cautious Frenchman needs help in getting on
• Card game is found at hotel - all feels a bit like what's left on the shelf
• Getting on one's horse, needing Monsieur to help in particular